The present invention relates to a photodiode which produces current in response to light, particularly ultraviolet light, and a photo IS using the photodiode.
With an increase in the amount of irradiation of ultraviolet light or rays due to the destruction of the ozone layer, the influence of ultraviolet light contained in sunlight on the human body and the environment has been feared in these days.
The ultraviolet light means invisible light in an ultraviolet range of 400 nm or less in wavelength. Since, however, the sunlight contains visible light and infrared rays in addition to the ultraviolet light, it has been required that an ultraviolet-light detecting photodiode separates and detects only the ultraviolet light.
Therefore, an ultraviolet-light penetration filter for allowing only ultraviolet light to pass therethrough is generally provided on the upper surface of a photodiode to separate and detect only the ultraviolet light.
In this type of ultraviolet-light penetration filter, the amount of ultraviolet transmission is reduced due to its deterioration. Therefore, as a conventional photodiode, a horizontal photodiode wherein comb teeth portions of an N+ diffusion layer in which an N-type impurity is diffused in a high concentration and which is shaped in the form of an “E”-shaped comb, and a P+ diffusion layer in which a P-type impurity is diffused in a high concentration and which is shaped in the form of a “π”-shaped comb, are engaged with each other and these comb teeth portions are disposed opposite to each other, is formed in a silicon semiconductor layer with the N-type impurity diffused therein in a low concentration, of a semiconductor wafer of SOI (Silicon On Insulator) structure in which the silicon semiconductor layer is formed on a silicon substrate with an embedded oxide film interposed therebetween. The thickness of the silicon semiconductor layer of the photodiode is set to a thickness of about 150 nm to allow visible light to pass therethrough and absorb only the ultraviolet light [refer to, for example, a patent document 1 (Japanese Unexamined Patent Publication No. Hei 7 (1995)-162024 (paragraph 0020 in the 3rd page and paragraph 0025 in the 4th page—paragraph 0035 in the 5th page, and FIGS. 2 and 3))].
In the above related art, however, the photodiode is formed wherein the silicon semiconductor layer is formed on the embedded oxide film of the semiconductor wafer of SOI structure, and the thickness of the silicon semiconductor layer of the photodiode is set to the thickness of about 150 nm, thereby to allow the visible light to pass therethrough and absorb only the ultraviolet light. Therefore, according to findings shown below, a problem arises in that the influence of reflection at the boundary face between the silicon semiconductor layer and the embedded oxide film cannot be avoided and the total quantity of ultraviolet light in the ultraviolet range of 400 nm or less in wavelength cannot be detected accurately.
That is, the inventors have determined by calculation the thickness of the silicon semiconductor layer which allows visible light to pass therethrough and absorb only a wavelength range of ultraviolet light, that is, which is capable of selectively detecting ultraviolet light.
That is, optical absorptance I/Io in silicon is represented by the Beer law expressed in the following equation (1):I/Io=exp(−αZ)  (1)where α: light absorption coefficient, Z: entrance depth of light, I: light intensity at depth Z, and Io: intensity of incident light
It is understood that when the optical absorptance I/Io is determined every thickness (Z) of the silicon semiconductor layer using the equation (1) in consideration of wavelength dependence of the light absorption coefficient α, and such a wavelength that the optical absorptance I/Io relative to the thickness of the silicon semiconductor layer 4 becomes 10%, is determined, the thickness of the silicon semiconductor layer may be set to a thickness of 50 nm or less to selectively have or hold sensitivity in the ultraviolet range of 400 nm or less in wavelength.
On the basis of the above result of calculations, a silicon semiconductor layer varied in various ways in a range of 50 nm or less in wavelength is formed in a semiconductor wafer of SOI structure, and horizontal photodiodes are formed in the silicon semiconductor layer. Sensitivity of these to the wavelength of light were measured by experiment.
FIG. 2 is a graph showing sensitivity of a photodiode at the time that the thickness of the silicon semiconductor layer is set to 40.04 nm.
It is understood that in the photodiode set to about 40 nm in thickness as shown in FIG. 2, a subpeak (marked in a circle shown in FIG. 2) exists in a wavelength range (purple) of visible light longer than the wavelength range of ultraviolet light of 400 nm or less in wavelength, and a photoelectric current that reacts to the wavelength range of visible light is contained in the detected photoelectric current.
It is considered that this is because in the case of an actual photodiode, although the above calculation is made assuming that light passes through the silicon semiconductor layer as it is, light is reflected by the boundary face between the silicon semiconductor layer and the embedded oxide film, and the length of a path through which the light passes becomes longer and hence it reacts with visible light having a wavelength longer than the wavelength range of ultraviolet light, so that it is absorbed into the silicon semiconductor layer and appears as a subpeak.
Such a subpeak further appears even at a thin silicon semiconductor layer. A result obtained by determining each wavelength (called “subpeak wavelength”) that appears thereat by experiment is shown in FIG. 3.
As shown in FIG. 3, the subpeak wavelength becomes shorter as the thickness of the silicon semiconductor layer becomes thinner. Assuming that the thickness of the silicon semiconductor layer is Tsi (unit: nm), and the subpeak wavelength is Ls (unit: nm), the subpeak wavelength is made approximate by the following experimental equation. In order to avoid the influence of reflection at the boundary face between the silicon semiconductor layer and the embedded oxide film and prevent reaction with visible light having a wavelength longer than a wavelength of 400 nm, i.e., avoid an error due to the addition of the photoelectric current in the wavelength range of visible light to the total quantity of the detected ultraviolet light, the thickness of the silicon semiconductor layer needs to be set to a thickness of 36 nm or less.Ls=2.457Tsi+312.5  (2)